A systematic analysis of the structure, function, and regulation of the pyruvate and alpha-ketoglutarate dehydrogenase complexes from microorganisms and from mammalian tissues is continuing. The mammalian pyruvate dehydrogenase complex contains a core, consisting of dihydrolipoyl transacetylase, to which pyruvate dehydrogenase (PDH), dihydrolipoyl dehydrogenase, and two regulatory enzymes (a kinase and a phosphatase) are joined. PDH can exist in a phosphorylated inactive form of a nonphosphorylated active form. The kinase which phosphorylates PDH is tightly bound to the transacetylase, whereas the phosphatase which dephosphorylates the phosphorylated PDH and activates it is loosely associated with the complex. Both Mg2 and Ca2 ions are required for activity of the phosphatase. It appears that in the presence of Ca2 ion the phosphatase binds to the transacetylase component of the complex and rapidly dephosphorylates and activates PDH in a Mg2 ion-dependent reaction. Experiments are in progress to gain further understanding of the physiological regulation of the activity of the kinase and the phosphatase and the mechanism of action of these two regulatory enzymes and to determine the nature of the binding of the kinase to the transacetylase and the number of kinase chains bound to the transacetylase. Systemic studies will be continued on the structures of the alpha-keto acid dehydrogenase complexes and on the reconstitution of these complexes from their component enzymes in order to gain further understanding of possible perturbations in the structures of the dihydrolipoyl transacetylase and the dihydrolipoyl transsuccinylase that apparently result in reduction of the symmetry of these core enzymes.